Rotary furnace having recycle provision

ABSTRACT

A rotary furnace apparatus, especially useful for carrying out a reaction of fluorspar and sulfuric acid, is provided with a mechanical recycling means within the furnace itself. The recycling means is fixed to an inner shell portion of the furnace so that it rotates with rotational movements applied to the furnace shell by a single driving means. In one embodiment, the recycling means is in the form of a screw conveying device having a cylindrical tube mounted coaxially within the furnace shell, and with two spiral paths defined within the cylindrical tube for receiving and conveying residue material from a downstream area within the furnace shell to an upstream area near the inlet end of the furnace. The screw conveyor is provided with a pair of scoops to pick up residue material and to deposit the material in controlled quantities into the two spiral paths defined in the conveyor. In another embodiment, the recycling means is in the form of an open-ended helical passageway which follows the inner cylindrical wall of the furnace shell from an area of residue pick up to the inlet end of the furnace.

United States Patent [72] Inventors William E. Watson Mount Tabor; Richard P. Troeger, Chatham, both of NJ. [211 App]. No. 886,613 [22] Filed Dec. 19, 1969 [45] Patented Sept. 21, 1971 [73] Assignee Allied Chemical Corporation New York, N.Y.

[54] ROTARY FURNACE HAVING RECYCLE PROVISION 18 Claims, 8 Drawing Figs.

[52] US. Cl 23/279, 23/153, 23/286, 263/33, 34/11, 34/129, 34/108 [51] Int. Cl B0lj 6/00, F 27b 7/ 16 [50] Field of Search 23/279, 153, 286; 263132, 33, 34;34/l08,l26, 129,11; 165/ 104 [56] References Cited UNITED STATES PATENTS l Jones.. N 263/33 X 4 3,396,953 8/1968 Sandbrook PrimaryExaminer-James I-I. Tayman, Jr. Attorneys-Albert L. Gazzola and Gerard P. Rooney ABSTRACT: A rotary furnace apparatus, especially useful for carrying out a reaction of fluorspar and sulfuric acid, is provided with a mechanical recycling means within the furnace itself. The recycling means is fixed to an inner shell portion of the furnace so that it rotates with rotational movements applied to the furnace shell by a single driving means. In one embodiment, the recycling means is in the form of a screw conveying device having a cylindrical tube mounted coaxially within the furnace shell, and with two spiral paths defined within the cylindrical tube for receiving and conveying residue material from a downstream area within the furnace shell to an upstream area near the inlet end of the furnace. The screw conveyor is provided with a pair of scoops to pick up residue material and to deposit the material in controlled quantities into the two spiral paths defined in the conveyor. In another embodiment, the recycling means is in the form of an openended helical passageway which follows the inner cylindrical wall of the furnace shell from an area of residue pick up to the inlet end of the furnace.

PATENTEBSEP21 1m SHEET 1 [1F 4 PATENTEU SEPZ? m1 SHEET [1F 4 ROTARY FURNACE HAVING RECYCLE PROVISION BACKGROUND AND BRIEF DESCRIPTION OF INVENTION This invention relates to improved apparatus useful for carrying' out a reaction of flubrspar and sulfuric acid, and the invention is particularly concerned with providing for a recycling means within a furnace reaction zone so that residue from a downstream portion of the reaction zone can be returned to the inlet end of the reaction zone.

In the past, the reaction has usually been carried out commercially in externally heated furnaces characterized by a metal shell provided with rails, scrapers, knockers, or other mechanical means to remove the byproduct incrustations which would otherwise form on the furnace walls, thus limiting the rate of heat transfer through the furnace walls into the furnace wall, so that a hard incrustation is invariably present which markedly reduces heat transfer. Furthermore, the resistance offered by the incrustation to the scraper causes very high torque on the scraper assembly,

leading to high maintenance costs and downtime. I b. Another type of furnace comprises rotating cylinders, ex-

ternally heated, horizontal or nearly so, provided with loose rails or heavy bars inside, which by rotation of the furnace, scrape and tumble against the furnace walls to remove incrustation. The constant exposure of fresh metal surfaces on the rails and furnace wall accelerates corrosion so that furnaces of this type require frequent replacement of the rails and furnace shells after operating for about 2,000 hours or less. c. A third type of furnace includes the use of heavy duty premixers which premix the calcium fluoride and sulfuric acid, either or both of which are usually preheated to attain partial reaction in the mixer, to insure that the feed to the furnace will not incrust the furnace wall. This arrangement does protect the furnace and eliminates the need for scraping devices, but the most corrosive part of the reaction is thereby transferred to the costly premixer. The fourth type of furnace is the one used to form a portion of the required sulfuric acid by reacting steam and sulfur trioxide within the furnace. Sulfuric acid in vapor phase (S0 will not react with fluorspar, but by showering the reaction mass through the sulfuric vapor, condensation and reaction are attained. While this system eliminates the problem of heat transfer through the furnace wall, it is relatively expensive to operate because of the cost of the sulfur trioxide, and the necessity for con structing the furnace shell of costly corrosion resistant alloys to resist the effect of the presence of sulfuric and hydrofluoric acid mixtures.

The above operations have certain drawbacks which seriously affect the economic feasibility of a commercial process-the first three have serious limitations when it is desired to design a high capacity furnace and the fourth is limited by high operating costs.

In our copending application, Ser. No. 876,552 filed Nov. 13, 1969, we have described a process wherein the above difficulties are obviated wherein hydrogen fluoride is produced by the reaction of calcium fluoride and sulfuric acid at temperatures between about 250 and 600 F. in the presence of more than 3 parts calcium sulfate per part of byproduct calcium sulfate produced. Preferably, the calcium sulfate is recycled byproduct calcium sulfate withdrawn at a point in the furnace prior to the completion of the reaction in the production of hydrogen fluoride.

The essential feature of that invention involves the use of more than 3 parts of calcium sulfate per part of byproduct calcium sulfate produced in the reaction mixture of fluorspar and sulfuric acid in the production of hydrofluoric acid and which is effectively achieved without any of the prior art difficulties. The effectiveness of this method of operation is surprising and unexpected since the furnace used may operate without any scrapers, knockers, or other means conventionally employed in removing incrustation from the shell. In operation, the mass within the furnace is substantially a free-flowing solid, and the walls are never exposed to the action of the highly corrosive liquid sulfuric acid. The HF produced is also of higher purity than that obtained from the types of furnaces previously discussed, because in those type furnaces the product gases contain large amounts of sulfur dioxide and elemental sulfur as a result of the reduction of sulfur acid by nascent hydrogen released during the attack or corrosion of the metal of the furnace by the acid. Elemental sulfur is particularly objectionable as it tends to deposit on heat transfer surfaces and to plug acid drip collection lines. 1

The present invention relates to a furnace system for the production of hydrogen fluoride which permits controlled recycling of residue within a single furnace without any requirement for external conveyors, multiple drives, or other complex and unreliable equipment. Thus, the improved apparatus of this invention provides a recycle conveying system within a single rotary furnace apparatus. A single drive motor can power the entire apparatus, including the internal recycle conveying system, and the recycle conveying system does not move relative to the furnace shell in which it is mounted, thereby eliminating problems of bearing supports or of erosion-accelerated corrosion between moving parts.

In one basic embodiment of the invention the recycling conveyor comprises a screw-conveying means in the form of a cylindrical conveyor tube having two spiral paths defined on its inner cylindrical wall. The two spiral paths are offset from each other by about and are of approximately the same pitch. The two paths are wound in a direction to carry residue material back towards the inlet end of the furnace apparatus when the furnace shell and the recycle conveyor are rotated in the same direction by a common drive means. Each of the two spiral paths of the screw conveyor communicates with a radially extending scoop means positioned at the downstream end of the recycle conveyor. The scoops are positioned to receive residue material from the reaction zone and to deposit controlled quantities of the material into their respective spiral paths with which they communicate. The scoops are preferably provided with screens to sort out oversized lumps of residue material so that large, hard lumps are not introduced into the recycle conveyor.

In another basic embodiment of the invention the recycle conveyor is in the form of a closed tube or passageway, having open ends, and which is positioned in a helical path around the inner cylindrical wall of the reaction zone portion of the furnace apparatus. At least one such helical passageway is provided in a rotary furnace so as to extend from a residue pickup area of the furnace back to an inlet end of the furnace. This embodiment is especially useful where it is desired to carry a recycle of residual calcium sulfate all the way back to the inlet end of a reaction zone which is receiving fluorspar and sulfuric acid as reactants. Also, the recycle conveyor of this embodiment functions as an effective metering device so that a desired recycle quantity can be accurately designed into the construction of a rotary furnace. The recycle conveyor of this embodiment may be combined with the screw-type conveyor of the first embodiment in any given rotary furnace apparatus.

Although the invention will be described with particular reference to a process for producing hydrogen fluoride, it will be recognized that the structural features and functions which will be discussed are applicable to rotary furnaces used in carrying out other treating processes. Additional details, features, and advantages of the invention will become apparent in the more detailed discussion below. In that discussion reference will be made to the accompanying drawings as briefly described below.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a side elevational view, somewhat schematic in its representation, of a rotary furnace apparatus of the type contemplated by the present invention;

FIG. 2 is an elevational view similar to FIG. l, but on an enlarged scale and showing only a portion of the FIG. 1 apparatus with respect to a first basic embodiment of a recycling means contained within the furnace apparatus;

FIG. 3 is an enlarged perspective view of the downstream end of the type of recycling means used in the embodiment shown in FIG. 2;

FIG. 4 is an enlarged sectional view taken on lines 44 of FIG. 2;

FIG. 5 is a sectional view similar to FIG. 4, but viewed from lines 55 of FIG. 2;

FIG. 6 is an enlarged top plan view of a downstream end portion of the recycle conveyor shown in FIG. 2, as seen generally on line 6--6 but with the outside furnace jacket omitted for clarity;

FIG. 7 is an enlarged elevational view of a rotary furnace incorporating a second embodiment of recycling means with the first embodiment recycling means of this invention; and,

FIG. 8 is an elevational view similar to FIG. 7, showing a rotary furnace provided with recycling means in accordance with the second embodiment of the invention.

DETAILED DESCRIPTION OF INVENTION Referring to FIG. 1, a typical rotary furnace apparatus of the type contemplated by this invention is depicted. The rotary furnace includes an inner, cylindrical furnace shell 10 into which reactants are introduced for producing hydrogen fluoride. Thus, the inner shell 10 functions as a reaction zone, and the reaction zone is preferably divided into two stages, with an upstream portion above an annular darn l2 and a downstream portion below the annular darn 12. The furnace shell 10 is supported for rotation by tires 14 and trunnions 16, in a manner well known in this art. A single driving means (not shown) of conventional structure is provided for rotating the furnace shell 10 through a bull or girth gear 18.

The illustrated apparatus is provided with a heating jacket around a major length of the furnace shell 10, and products of combustion are circulated through the heating jacket 20 by a fan means contained within a housing 22. The products of combustion are generated in a combustion chamber 24 in which fuel gas or oil is burned with a controlled amount of air. Known means are provided for controlling the introduction of fuel and air into the combustion chamber 24. In a typical hydrogen fluoride process, combustion gases are generated at temperatures below 1300 F. at their point of admission 26 to the heating jacket 20. Due to the volume of combustion gas which can be circulated by the fan 22, there is a temperature drop from the point of admission 26 to the exits 28 from the jacket of approximately 300 F. Dampers 30 are provided in return pipes 32 to regulate the flow of hot gases to each end of the heating jacket 20. The pressure of gases within the heating jacket 20 is controlled to be maintained at essentially atmospheric pressure to avoid excessive hot gas loss or cold air in-leakage across crude seals 34 provided at each end of the jacket.

The furnace shell 10, in which reaction takes place, is provided with end walls at each end so that it is a substantially closed furnace. The entire furnace assembly, including the furnace shell 10, is canted slightly so that its inlet end A is slightly higher than its outlet end B. This provides for a movement of reactants from the inlet end to the outlet end while the furnace is being rotated.

FIG. 2 illustrates additional details of the inlet end of the furnace apparatus. The end is closed and sealed by an upright wall or head 40 which is secured to the cylindrical furnace shell 10 so as to rotate therewith. The end wall 40 is preferably recessed into the furnace shell 10 for a sufficient distance to be included in the area of the furnace shell 10 which is surrounded and heated by the heating jacket 20. Since a tire 14 is provided for mounting the inlet end of the furnace on a trunnion 16, it is not possible to extend the heating jacket all the way to the very end of the furnace shell 10. Accordingly, it is preferred to define the inlet end of the reaction zone by an upright wall 40 which positions the reaction zone completely within a heated area of the furnace shell 10. This is an important feature of the apparatus because any unheated sections in the reaction zone could be sources of condensation of corrosive liquids within the furnace shell. Reactants are introduced into the reaction zone through the upright wall 40. The means for introducing reactants are conventional and include a screw feeder 42 for feeding fluorspar into the reaction zone together with an acid pipe 44 for feeding sulfuric acid into the reaction zone. There is no premixing of the fluorspar and acid prior to being mixed with recycled residue within the reaction zone, thereby avoiding the formation of a sticky mass at the inlet end of the furnace. The screw feeder and pipe are mounted in a stationary housing 46 which is provided with a seal at 48. A rotating portion 50 of the housing is secured to the end wall 40 and is sealed in its movement relative to the stationary housing 46. Hydrogen fluoride gas is removed from the system through the wall 40 and the stationary housing 46 by way of a flue 52. This general arrangement for feeding reactants into a reaction zone and removing I-IF gas therefrom are generally known in this art and do not constitute a separate part of the present invention.

Referring back to FIG. 1, the downstream end of the furnace shell 10 is provided with internal lifters 54 which lift and dump calcium sulfate residue material from the end of the final reaction zone into a stationary chute 56. The stationary chute 56 is provided with a conventional sealing device which has a rotating star valve 58 to prevent loss of HF gas out of the furnace through the stationary chute. Also, the valve 58 prevents an excess of air from being sucked into the furnace, although the furnace reaction area is maintained at nearly atmospheric, or slightly below atmospheric, pressure during operation.

In accordance with the invention at least one recycling means is positioned within the furnace shell 10 to pick up a portion of residue material from a downstream reaction area of the furnace and to return the material back to the inlet end of the furnace. Preferably, the recycling means is positioned to pick up residue material from the downstream end of the initial reaction zone defined above the annular dam 12 so that a preferred ratio of residue is recycled back to the inlet end of the furnace prior to final reaction of the mixture in the second reaction zone below the annular dam 12. A preferred process for reacting fluorspar with sulfuric acid provides for a recycling in excess of three parts of residue material for every part of calcium sulfate produced in the reaction. The residue material not recycled is moved over the annular dam l2 and passed into the discharge end of the reaction zone. The invention will be further described below with reference to two basic forms of recycling means which may be positioned within the furnace shell 10 for returning preferred ratios of calcium sulfate residue back to the inlet end of the furnace. FIGS. 1-6 relate to constructional features of a first basic embodiment of a recycling means in accordance with the present invention, FIG. 8 represents the second basic embodiment and FIG. 7 illustrates a preferred arrangement wherein the first embodiment of recycling means is combined with the second embodiment of recycling means. The first embodiment will be referred to hereafter as a screw-type-recycling means, and the second embodiment will be referred to as a worm-typerecycling means in the form of an open-ended, closed helical passageway formed about the inner cylindrical wall of the furnace shell. Preferably, a furnace system will combine both of the two types of recycling means into a single construction, but for purposes of clarity of description, the two recycling means will be described below separately.

Screw-Type-Recycling MEANS Referring to FIGS. 1 and 2, a screw-type-recycling means 60 is shown as being mounted in a fixed position within the furnace shell so that the entire recycling means rotates with rotational movements applied to the furnace shell 10 and without any requirement for separate driving arrangements. The screw-recycling means 60 includes a cylindrical conveyor tube 62 mounted coaxially within the furnace shell 10, and the conveyor tube 62 contains two spiral or helical paths defined on its inner cylindrical wall surface. The spiral paths are,

defined by two elongated flight elements 64 helically wound about the interior surface of the tube 62 to form the required two spiral paths within the tube 62. The elongated elements 64 are offset from each other by 180 and are of the same pitch. Both spiral paths are wound in a direction to carry residue material back towards the inlet end of the furnace shell when the furnace shell and the screw-recycling means 60 are rotated together in the same direction of rotation. In this sense, the screw-recycling means is constructed to function something like an Archimedes screw conveyor for carrying residue material from a downstream area within the furnace to an upstream area within the furnace. The cylindrical conveyor tube 62 is mounted within the furnace by any suitable means, such as by posts 66 positioned about its external surface, Thus, when material is deposited into the two spiral paths of the rotating-screw-recycling means 60, it is carried back towards the inlet end of the furnace, where it is dropped from an open end 68 of the conveyor tube 62. It is desirable to place the open end 68 of the recycling means as close as possible to the inlet end of the reaction zone so that reactants are mixed with recycled residue. However, it can be seen that the end of the conveyor tube 68 cannot interfere with apparatus used for the introduction of reactants into the inlet end of the reaction zone, and therefore, there is a practical limit, in the illustrated equipment, to the length of the cylindrical conveyor tube 62 that can be used.

Since the conveyor tube 62 is mounted along the central longitudinal axis of the reaction zone, so as to not interfere with movement of a bed of residue from the inlet end of the furnace towards the outlet end, it is necessary to provide means for lifting quantities of residue material into the two spiral paths defined within the screw-conveying tube 62. The invention provides for a pair of scoop means 70 mounted at the downstream end of the screw-conveying tube 62 for picking up residue material and for depositing controlled quantities of the material into the two spiral paths defined within the screw-conveying means. As shown in FIGS. 3-5, the pair of scoop means 70 are generally in the form of radially extending boxlike structures displaced form one another by 180 to provide a communication between the downstream end of the screw-conveying tube 62 and the inner wall surfaces of the furnace shell 10.

This invention has discovered that recycling of residue can be carefully controlled by establishing certain relationships between the scoop means 70 and the screw-conveying means contained within the cylindrical tube 62. As shown in FIG. 3, each of the scoop means 70 is in the general fonn of a boxlike structure having an open inlet end 72. The open inlet of each scoop means is directed transversely across the furnace shell so that residue material being moved along within the shell can be received into the inlets 72 as the shell and recycling means 60 are rotated. FIG. 2 shows the inlet ends 72 as viewed transversely across the furnace apparatus.

Each of the inlets 72 may be provided with screening means 74 in the form of a plurality of rods for screening out oversized lumps of residue. This prevents a scooping up and depositing of oversize lumps into the screw flights. of the conveying means. However, in some applications the screening means 74 can be omitted, and the scoop means can be provided with completely open inlets 72. The screening means 74 may be set at an oblique angle relative to the longitudinal axis of the furnace, as shown in the FIG. 6 view.

It can be appreciated from the displacement of the two scoop means that one scoop means functions to pick up residue material from the bottom of the furnace shell, at a given time during rotation of the shell and the recycling means, while the other scoop means deposits its load of residue material into the downstream end of the conveying tube 62. Thus, as the entire assembly rotates, the two scoop means function to alternately plow through the bed of residue material and to pick up a quantity of material for dumping into the screw-conveying means 60. The annular dam 12 functions to cause a sufficient buildup of residue bed within the furnace to allow the scoop means 70 to receive desired quantities for recycle.

Each scoop means 70 is provided with an inclined slide 74 for depositing its load of residue into the end of the conveyor tube 62, and the relationships of the two inclined slides 74 to the end of the conveyor tube 62 are important to a successful operation of the recycling means. The general relationship of each inclined surface 74 to the end of the conveyor tube 62 is shown in FIG. 2. Each of the inclined surfaces 74 communicates with one of the spiral paths defined within the conveyor tube 62, although an overflow provision is made so that the quantity of material deposited into each spiral path can be controlled. FIG. 3 further illustrates the positions of the inclined slides 74 as related to the end of the conveyor tube 62. It can be seen that the slides 74 define a closed end to the conveyor tube 62 so that there is a path of communication with the inlet opening 72 of each scoop means 70. The scoop means 70 are further provided with wall sections 80, 82, and 84 (see FIG. 4) to form closed boxlike structures within the furnace shell 10. These wall sections are affixed to a section 86 of the inner surface of the furnace shell to enclose the scoop means, except for their inlets 72 and their points of entry into the conveyor tube 62. The screening section of each inlet may be closed off by a plate section 87 at its connection to an extended part of the cylindrical tube 62, however, the plate 87 can be omitted if desired.

FIG. 4 is a view from the inlet end of the furnace shell 10, showing the downstream end of the recycling means 60 at its point of communication with the scoop means 70. As illustrated, each of the two elongated flight elements 64, which together make up the two spiral screw paths defined within the conveyor tube 62, originates along a diameter 88 of the conveyor tube. The line of origination of the two elements 64 is defined by the line of juncture between the two inclined slides 74 of the scoop means 70. Thus, each of the two separate spiral paths defined by the flight members 64 has an origination line which is aligned with an inclined surface 74 of one of the scoop means 70. This arrangement is quite important to a recycling means incorporating two spiral paths and utilizing two scoop means of the type disclosed in this specification.

FIG. 4 also illustrates the paths of movement followed by residue material within the recycling means when the furnace shell 10 and the recycling means 60 are rotated in the direction of the arrow. When one of the scoop means 70 approaches the bed of residue material during rotation of the furnace shell, material is picked up through the inlet 72 of the scoop means. Then, as the shell, and the recycling means therein, continue to rotate, the picked-up material falls back into a closed section of the scoop means from where it can drop down onto its associated inclined slide 74 for being deposited into the conveyor tube 62. FIG. 4 shows an arrow pointing downwardly in the interior of the upper scoop means 70 to represent the fall of material down towards the upper inclined slide 74. However, it can be appreciated that the fall of material actually begins while the scoop means is at some intermediate position between the bottom and the top of the furnace, and material can begin to slide down the sidewall 82 of the uppermost scoop means before it reaches a straight-up position. Each scoop means 70 communicates with the conveyor tube 62 only for the width of its wall section 80, the remainder of the scoop means (mainly, the triangular inlet portion which includes the screening bars 72) is closed off from the conveyor tube 62 be extended portions of the conveyor tube itself (see FIG. 3). Thus, residue material descends into the end of the screw-conveying means along the left half of the upper inclined slide when the scoop means is in the position shown in FIG. 4.

FIG. 4 also shows two separate arrows extending downwardly over the inclined slide 74 which is associated with the upper scoop means 70. The left arrow illustrates the flow of material onto a flight element 64 which is generally aligned with the upper inclined slide 74. Thus, a portion of the downward flow falls into one of the two spiral paths defined in the conveyor tube 62. However, should an excess of material fall onto the flight element 64 (the left-hand element of FIG. 4 when the scoops are in the positions indicated), there is an automatic overflowing of the excess down into the opposed scoop means 70 positioned at the bottom of the apparatus. FIG. 4 also shows a downwardly directed arrow in the center area of the upper inclined slide 74, and this arrow represents excess material from the slide falling behind the same flight element 64 and back into the opposed scoop means 70 at the bottom of the apparatus. Thus there is an automatic compensation for differing amounts of material that are picked up by the individual scoop means 70, and the spiral paths are filled only by a predetermined amount which can be easily conveyed back to the inlet end of the furnace, thereby regulating the desired ratio of material to be recycled. As the apparatus shown in FIG. 4 continues to rotate, the deposited quantity of material in the lower spiral path is fed towards the inlet end of the reaction zone, and the recycle process repeats itself when the illustrated lower scoop means arrives near the top of the apparatus. Each complete rotation advances residue material for one pitch in the tube 62 so that an empty pocket in the spiral path is presented to each scoop means when it rotates to the top position. By relating the opposed scoop means and the opposed origins for the flight elements 64, as discussed above, a control of recycle quantity can be easily designed into any given apparatus. Preferably, the scoop means 70 are designed to always pick up an amount of material which exceeds the quantity necessary to feed the screw flights to full capacity. This allows a complete filling of the recycle conveyor, with an overflowing of excess material into an opposed scoop. The overflow material is then deposited in the screw conveyor in the next one-half cycle.

The structural elements which make up the recycle means are fabricated from known materials which can resist the corrosive environment of the furnace, and all elements are assembled together by known techniques. For example, the individual spiral flight elements 64 can be welded into position within the conveyor tube 62, and their end portions at 90 can be welded along the juncture line 88 of the two inclined slides 74.

Worm-Type'Recycling Means FIGS. 7 and 8 illustrate a second embodiment of the recycling means of this invention. FIG. 7 shows the second embodiment recycling means combined with the first embodiment recycling means in a preferred arrangement for a hydrogen flouride-process furnace. FIG. 8 illustrates a furnace structure utilizing only the second embodiment type of recycling means.

The basic furnace arrangement shown in FIGS. 7 and 8 is the same as that previously described with reference to FIGS. I and 2 above.

The second embodiment of this invention comprises a recycling means which may be considered a worm type of recycle conveyor having at least one helical passageway 100 formed about the inner cylindrical wall of the furnace shell 10.

The helical passageway is open-ended at its inlet end I02 and at its outlet end 104, but the remainder of the passageway is closed off from the interior of the furnace shell 10. The helical passageway 100 may be formed around the inner cylindrical wall of the furnace shell in any suitable manner. For example, a semicircular tunnel element may be welded, or otherwise secured, about the inner furnace wall so as to provide the type of configuration illustrated in FIGS. 7 and 8.

In the FIG. 7 arrangement, the recycling means 100 is combined with the screw type of recycling means 60 discussed above. Such a combination is particularly advantageous because the screw-type-recycling means 60 can handle larger quantities of recycle material to bring the recycle ratio up to a desired level whereas the worm-type-recycling means 100 can introduce recycle material to the extreme end of the reaction zone at the very point of introduction of reactants into the furnace shell. The recycling means 60 cannot extend far enough towards the end wall 40 of the furnace shell to dump recycle material at the introduction point of reactants, and therefore, the second embodiment of recycling means assists in preventing a formation of a sticky, corrosive mass at the inlet end of the furnace by depositing recycle residue through its outlet 104 at the extreme end of the reaction zone. Another advantage of the recycling means 100 of this embodiment is that it deposits recycle material back into the charge end of a reaction zone without dropping the recycle material through the furnace atmosphere within the furnace shell 10. A dropping of material through the atmosphere of the reaction zone, especially adjacent to the point at which I-IF gas is to be withdrawn, can create a dust problem in the takeoff ducting for the HF gas. Thus, the recycling means 60 of the first embodiment cannot deposit recycle calcium sulfate as close to the inlet end of a furnace as can the second embodiment recycling means 100.

The recycling means 100 functions by receiving a quantity of residue material into its inlet end 102 as it rotates with rotational movements of the furnace shell 10. Continued rotation of the shell and of the recycling means causes the received residue material to be carried back towards the inlet end of the furnace until it is discharged from the end 104.

FIG. 8 illustrates an arrangement wherein two separate recycling means 100 are formed around the inner cylindrical wall of a furnace shell so as to deposit recycle residue material from positions at the inlet end of the furnace. Each of the two recycling means 100 are of the same pitch and direction so that they follow each other around the inner wall of the furnace shell 10 in a parallel relationship to each other. Both of the recycling means 100 receive residue material at their open inlet ends 102 as their respective inlet ends plow through an accumulation of residue material upstream of the annular dam 12. This initiates recycling of the residue material back towards the inlet end of the furnace.

As with the first embodiment, the recycling means 100 can function as an effective metering device to control the quantity of recycle material. The dimensions of the recycling means 100 can be set to return desired portions of residue to the inlet end of the furnace.

Having described the constructional features of the two basic embodiments of this invention, it can be seen that an improved arrangement for recycling material within a furnace type of apparatus has been provided. Although the invention has been described with reference to specific embodiments, it can be appreciated that the principles and concepts disclosed herein can be practiced with various equivalent or obvious in view of this invention, are intended to be included in the scope of the invention, as claimed below.

We claim:

1. In a rotary furnace apparatus for carrying out a reaction of fluorspar and sulfuric acid to produce hydrogen fluoride, the combination comprising: a cylindrical furnace shell for receiving and reacting the fluorspar with sulfuric acid, said furnace shell having end walls and said shell being mounted with its axis slightly inclined so that reactants will move from the first end to a second end of the furnace shell; means for rotating and heating said furnace shell; inlet means for introducing reactants into said first end of the furnace shell; outlet means for removing hydrogen fluoride from the reaction zone and separate outlet means at said second end for removing final reaction residue from the furnace shell; and internal recycling means for picking up and recycling a portion of residue material from a point within the furnace shell back to the area of said first end, said recycling means comprising screw-conveying means which comprises a cylindrical conveyor tube mounted coaxially within said furnace shell, said conveyor tube containing two spiral paths defined on its inner cylindrical wall, said spiral paths being offset from each other by 180 and of the same pitch, and said spiral paths being wound in a direction to carry residue material back towards the inlet ends of the furnace shell when the furnace shell and the screw-conveying means are rotated in the same direction, and including scoop means at the downstream end of said screw-conveying means to pick up residue material from the inner wall area of said furnace shell and for depositing controlled quantities of material into said two spiral paths of said screw-conveying means.

2. The combination of claim 1 wherein said recycling means has a capacity to recycle to the area of said first end at least three parts of residue material for every part of residue material which is produced.

3. The combination of claim 1 wherein said furnace shell generally includes a first reaction zone which is upstream of a second reaction zone, and wherein said recycling means is positioned to pick up residue material from the downstream end of said first reaction zone for recycling back to the upstream end of the same reaction zone.

4. The combination of claim 1 wherein said screw-conveying means is positioned within said furnace shell so that its axis of rotation is coincident with the axis of rotation of said furnace shell.

5. The combination of claim 4 wherein said screw-conveying means is carried for rotation with rotational movements of the furnace shell, and said screw-conveying means and said furnace shell being rotated by a common means.

6. The combination of claim 1 wherein two scoop means are provided in 180 positions from each other, with each of said two scoop means having an open inlet end for receiving residue material and an open outlet end communicating with the downstream open end of said cylindrical conveyor tube.

7. The combination of claim 6 wherein the downstream end of said cylindrical conveyor tube is provided with two inclined surfaces, each of which communicates with one of said scoop means, and both inclined surfaces being joined along a line taken at a diameter of said cylindrical conveyor tube.

8. The combination of claim 7 wherein said two spiral paths each originate along the line of juncture of said two inclined surfaces.

9. The combination of claim 6 wherein the open inlet end of each of said scoop means is provided with a screening means to sort out oversized lumps of residue material.

10. The combination of claim 1 and including a second recycling means in the form of an open-ended, closed helical passageway formed about the inner cylindrical wall of said fur nace shell.

11. The combination of claim 10 wherein said second recycling means extends at least to a point within the area of the first end of the furnace as the first recycling screw-conveying means.

12. The combination of claim 1 wherein at least the end wall at the inlet end of the furnace shell is recessed into said furnace shell for a sufficient distance to be included in an area of said furnace shell which is heated by a cylindrical jacket surrounding said furnace shell.

13. In rotary furnace apparatus which is especially useful for carrying out a reaction of fluorspar and sulfuric acid, the improvement comprising:

recycling means contained within a furnace shell portion of said apparatus for picking up a portion of residue material from a downstream area of the shell and returning the residue material to an upstream area of the shell, said recycling means being fixed in its position relative to the furnace shell so that it will rotate with rotational move- 5 ments, of the furnace shell, and said recycling means being in the form of a screw-conveying means having a cylindrical conveying tube mounted coaxially within said furnace shell, said conveyor tube containing two spiral paths defined on its inner cylindrical wall, said spiral paths being ofiset from each other by 180 and of the same pitch, and said spiral paths being wound in a direction to carry residue material back towards the inlet end of the furnace shell when the furnace shell and the screw-conveying means are rotated in the same direction, and including scoop means at the downstream end of said screw-conveying means to pick up residue material and for depositing controlled quantities of material into said two spiral paths of said screw-conveying means.

14. In a rotary furnace apparatus which is especially useful for carrying out a reaction of fluorspar and sulfuric acid, the improvement comprising:

separate first and second recycling means contained within a furnace shell portion of the apparatus for returning residue material from a downstream area in the furnace shell back to an upstream area in the furnace shell, each of said recycling means being fixed in its position relative to the furnace shell so that it rotates with rotational movements of the furnace shell.

15. The improvement of claim 14 wherein said first recycling means comprises a screw-conveying means defined within a cylindrical conveying tube mounted coaxially within said furnace shell, said conveying tube containing two spiral paths defined on its inner cylindrical wall, said spiral paths being offset from each other by 180 and of the same pitch, and said spiral paths being wound in a direction to carry residue material back towards the inlet end of the furnace shell when the furnace shell and the screw conveying means are rotated in the same direction, and including scoop means at the downstream end of said screw-conveying meansto pick up residue material and for depositing controlled quantities of material into said two spiral paths of said screw-conveying means.

16. The improvement of claim 14 wherein said second recycling means comprises at least one open-ended, closed, helical passageway formed around the inner cylindrical wall of said furnace shell.

17. In a rotary furnace apparatus for carrying out a reaction of fluorspar and sulfuric acid to produce hydrogen fluoride, the combination comprising: a cylindrical furnace shell for receiving and reacting fluorspar with sulfuric acid, said furnace shell having end walls and said shell being mounted with its axes slightly inclined so that the reactants will move from a 55 first end to a second end on the furnace shell; means for rotating and heating said furnace shell; inlet means for introducing reactants into said first end of the furnace shell; outlet means for removing hydrogen fluoride from the reaction zone and separate outlet means at said second end for removing final reaction residue from the furnace shell; and internal recycling means for picking up and recycling a portion of residue material from a point within the furnace shell back to the area of said first end, said recycling means comprising two openended CLOSED, helical passageways formed about the inner cylindrical walls of said furnace shell along a path which is displaced 180 from the path of the other helical passageway and extending from an area of residue pickup to said first end of the furnace.

18. In a rotary furnace apparatus which is especially useful '70 for carrying out a reaction of fluorspar and sulfuric acid the improvement comprising: internal recycling means contained within a furnace shell portion of said apparatus for picking up residue material from a downstream area of the furnace shell and returning the residue material to an upstream area of the furnace shell, said recycling means being fixed in its position which is displaced 180 from the path of the other helical passageway and extending from an area of residue pickup to said first end of the furnace. 

2. The combination of claim 1 wherein said recycling means has a capacity to recycle to the area of said first end at least three parts of residue material for every part of residue material which is produced.
 3. The combination of claim 1 wherein said furnace shell generally includes a first reaction zone which is upstream of a second reaction zone, and wherein said recycling means is positioned to pick up residue material from the downstream end of said first reaction zone for recycling back to the upstream end of the same reaction zone.
 4. The combination of claim 1 wherein said screw-conveying means is positioned within said furnace shell so that its axis of rotation is coincident with the axis of rotation of said furnace shell.
 5. The combination of claim 4 wherein said screw-conveying means is carried for rotation with rotational movements of the furnace shell, and said screw-conveying means and said furnace shell being rotated by a common means.
 6. The combination of claim 1 wherein two scoop means are provided in 180* positions from each other, with each of said two scoop means having an open inlet end for receiving residue material and an open outlet end communicating with the downstream open end of said cylindrical conveyor tube.
 7. The combination of claim 6 wherein the downstream end of said cylindrical conveyor tube is provided with two inclined surfaces, each of which communicates with one of said scoop means, and both inclined surfaces being joined along a line taken at a diameter of said cylindrical conveyor tube.
 8. The combination of claim 7 wherein said two spiral paths each originate along the line of juncture of said two inclined surfaces.
 9. The combination of claim 6 wherein the open inlet end of each of said scoop means is provided with a screening means to sort out oversized lumps of residue material.
 10. The combination of claim 1 and including a second recycling means in the form of an open-ended, closed helical passageway formed about the inner cylindrical wall of said furnace shell.
 11. The combination of claim 10 wherein said second recycling means extends at least to a point within the area of the first end of the furnace as the first recycling screw-conveying means.
 12. The combination of claim 1 wherein at least the end wall at the inlet end of the furnace shell is recessed into said furnace shell for a sufficient distance to be included in an area of said furnace shell which is heated by a cylindrical jacket surrounding said furnace shell.
 13. In rotary furnace apparatus which is especially useful for carrying out a reaction of fluorspar and sulfuric acid, the improvement comprising: recycling means contained within a furnace shell portion of said apparatus for picking up a portion of residue material from a downstream area of the shell and returning the residue material to an upstream area of the shell, said recycling means being fixed in its position relative to the furnace shell so that it will rotate with rotational movements, of the furnace shell, and said recycling means being in the form of a screw-conveying means having a cylindrical conveying tube mounted coaxially within said furnace shell, said conveyor tube containing two spiral paths defined on its inner cylindrical wall, said spiral paths being offset from each other by 180* and of the same pitch, and said spiral paths being wound in a direction to carry residue material back towards the iNlet end of the furnace shell when the furnace shell and the screw-conveying means are rotated in the same direction, and including scoop means at the downstream end of said screw-conveying means to pick up residue material and for depositing controlled quantities of material into said two spiral paths of said screw-conveying means.
 14. In a rotary furnace apparatus which is especially useful for carrying out a reaction of fluorspar and sulfuric acid, the improvement comprising: separate first and second recycling means contained within a furnace shell portion of the apparatus for returning residue material from a downstream area in the furnace shell back to an upstream area in the furnace shell, each of said recycling means being fixed in its position relative to the furnace shell so that it rotates with rotational movements of the furnace shell.
 15. The improvement of claim 14 wherein said first recycling means comprises a screw-conveying means defined within a cylindrical conveying tube mounted coaxially within said furnace shell, said conveying tube containing two spiral paths defined on its inner cylindrical wall, said spiral paths being offset from each other by 180* and of the same pitch, and said spiral paths being wound in a direction to carry residue material back towards the inlet end of the furnace shell when the furnace shell and the screw-conveying means are rotated in the same direction, and including scoop means at the downstream end of said screw-conveying means to pick up residue material and for depositing controlled quantities of material into said two spiral paths of said screw-conveying means.
 16. The improvement of claim 14 wherein said second recycling means comprises at least one open-ended, closed, helical passageway formed around the inner cylindrical wall of said furnace shell.
 17. In a rotary furnace apparatus for carrying out a reaction of fluorspar and sulfuric acid to produce hydrogen fluoride, the combination comprising: a cylindrical furnace shell for receiving and reacting fluorspar with sulfuric acid, said furnace shell having end walls and said shell being mounted with its axes slightly inclined so that the reactants will move from a first end to a second end on the furnace shell; means for rotating and heating said furnace shell; inlet means for introducing reactants into said first end of the furnace shell; outlet means for removing hydrogen fluoride from the reaction zone and separate outlet means at said second end for removing final reaction residue from the furnace shell; and internal recycling means for picking up and recycling a portion of residue material from a point within the furnace shell back to the area of said first end, said recycling means comprising two open-ended CLOSED, helical passageways formed about the inner cylindrical walls of said furnace shell along a path which is displaced 180* from the path of the other helical passageway and extending from an area of residue pickup to said first end of the furnace.
 18. In a rotary furnace apparatus which is especially useful for carrying out a reaction of fluorspar and sulfuric acid the improvement comprising: internal recycling means contained within a furnace shell portion of said apparatus for picking up residue material from a downstream area of the furnace shell and returning the residue material to an upstream area of the furnace shell, said recycling means being fixed in its position relative to the furnace shell so that it will rotate with rotational movement of the furnace shell, said recycling means comprising two open-ended, closed, helical passageways formed about the inner cylindrical wall of said furnace shell along a path which is displaced 180* from the path of the other helical passageway and extending from an area of residue pickup to said first end of the furnace. 